The present invention refers to a process for the production of hydrogen and electrical energy from reforming of bioethanol, with the use of fuel cells and zero emission of pollutants. Ethanol is produced from biomass which contains sugar and/or cellulosic components, originating from any source. Aqueous solution of ethanol (40-70% by weight) is mixed with air (0-0.5 mol oxygen per mol ethanol) and is fed to a reactor which contains suitable catalyst so as for the reactions of partial oxidation and reforming of ethanol to take place. In the same or a different reactor the shift reaction for the consumption of carbon monoxide and further production of hydrogen is taking place.
The gaseous mixture which is produced in this manner is rich in hydrogen which can be separated and used in different applications. Alternatively, the gaseous mixture is fed to a fuel cell, preferably of the phosphoric acid or proton exchange membrane or solid polymer, in which electrical energy and heat are produced.
No emissions harmful to the environment are produced in any of the stages of the above process.
The use of biomass as a renewable energy source has been investigated and proposed for many years [1]. Three different means of use of biomass as energy source have been applied internationally: combustion, pyrolysis for the production of gaseous and liquid fuels, and fermentation for the production of ethanol. Sources of biomass can be plants which have certain specific characteristics and which are grown for this purpose, or waste materials from cultivation of edible products or from agroindustries or from forestry. Studies which have been conducted in recent years show that there are significant quantities and sources of biomass which can be utilized for the production of energy.
The production of ethanol from biomass which is often referred to in the bibliography as xe2x80x9cbio-ethanolxe2x80x9d, is known and is practiced in large scale, mostly in North and South America and in Europe [2]. The processes for the production of ethanol can be classified in two large categories: those which utilize sugar-containing raw materialsxe2x80x94products of energetic cultivations (for example sweet sorghum) and those which utilize cellulosic raw materials originating from energetic cultivations (sorghum, cane, solid residue of sweet sorghum, etc.) as well as from residues of agroindustries. In the first case sugars are directly fermented for the productions of ethanol while in the second case the hydrolysis step or other processes are proceeded for the production of sugars which are then converted to ethanol via fermentation [1,2].
Although the technology of biomass fermentation for the production of ethanol is mature, it has not been applied in large scale, at least in Europe, for economic reasons. A large fraction of the cost of bio-ethanol is the cost of separation of the aqueous solution which derives from fermentation and which contains approximately 8 to 12% ethanol. In order to use ethanol as a fuel in internal combustion engines the required purity exceeds 99%. Because ethanol and water form an azeotropic solution when the ethanol content is approximately 95%, further purification requires energy-consuming techniques, the application of which increases significantly the cost of bio-ethanol.
Purpose of the present invention is to utilize ethanol which is produced from biomass for the production of energy without the requirement of its separation from water to a large degree. With the new method which is presented, the cost of ethanol production is reduced significantly while, simultaneously, the thermodynamic efficiency of its use is increased significantly (with the application of fuel cells) and the gaseous pollutants which are produced during its burning (for example in internal combustion engines) are eliminated.
According to the present invention, a mixture of ethanol, oxygen and water react and produce hydrogen and carbon dioxide, and the hydrogen is fed to a fuel cell which produces electrical energy from the electrochemical oxidation of hydrogen to water. The reforming of ethanol with water which is described by the reaction:
C2H5OH+3H2Oxe2x86x922CO2+6H2, xcex94H=+23.7 KJ/Mol xe2x80x83xe2x80x83(1)
has been little investigated, in contrast to the reforming of methanol. It has been reported that reforming of ethanol over a copper catalyst of a Pd/ZnO produces acetic acid, acetaldehyde, H2 and heavier oxygen-containing products [3]. Acetaldehyde is also produced when the reaction is taking place on Ni and Pt catalyst supported on MgO[4]. Complete reforming of ethanol on Ni catalyst has been reported, which, however, deactivates rapidly, probably due to carbon deposition [5]. The reaction of reforming of ethanol with water (Reaction 1) is endothermic and consequently supply of heat to the reactor is required. The simplest way to achieve this is the co-feeding of oxygen (air) together with the ethanol-water mixture over a suitable catalyst so as a small fraction of ethanol to be oxidized towards CO2 and H2O, producing the required heat for the reforming reactions:
C2H5OH+xc2xdO2+2H2Oxe2x86x925H2+2CO2, xcex94H=xe2x88x9220 Kcal/molxe2x80x83xe2x80x83(2)
As far as we know, no study has appeared in the literature referring to the partial oxidation/reforming of ethanol.
According to the invention which is presented the hydrogen which is produced from the partial oxidation/reforming of ethanol, is fed to a fuel cell for the production of electrical energy. The advantages of fuel cells are the zero emission of pollutants since the only product of combustion of hydrogen is water and the improved thermodynamic efficiency in comparison with internal combustion engines.
Fuel cells are an innovation which finds applications in the production of electrical energy without the emission of pollutants, consuming hydrogen as fuel which is oxidized electrochemically with oxygen, with simultaneous production of electrical energy [6]. The efficiency of fuel cells is approaching 70% of the heat which corresponds to the combustion of hydrogen, i.e., it is twice the efficiency of thermal engines which is subjected to the thermodynamic constraints of the Carnot type. There are five types of fuel cells which differ in the type of electrolyte and in the temperature of operation: 1) Phosphoric acid which is the most commercially advanced type and operate at temperatures around 200xc2x0 C. They produce electrical energy with efficiency from 40 to 80% [7]. 2) Proton exchange membranes or solid polymers which operate at low temperatures, approximately 100xc2x0 C., and offer large power density with quick response to power demands [8]. 3) Solid Oxide Fuel Cells which can be utilized in applications in which large power is required and operate at about 1000xc2x0 C. [9]. 4) Molten Carbonate Salts, whose electrolyte consists of molten carbonate salts of Li and K, the fuel is hydrogen and CO and the oxidizing mixture consists of oxygen and CO2. They operate at temperatures of 600xc2x0 C. with a large efficiency of fuel over power production [9]. 5) Alkalines which were used in space applications and produced power with an efficiency of approximately 70% operating at temperatures 60-150xc2x0 C.